1. Field of the Invention
This invention is directed to radial pneumatic tires for vehicles and in particular to the conductivity of a tire having a tread with a high silica content.
2. Description of the Art
The electrostatic charge on a vehicle with respect to a zero ground voltage is dissipated by the vehicle's tires being in contact with the ground. This electrostatic charge is produced by the mechanical and electrical components within the vehicle and aerodynamic friction as well as the tire to ground contact during vehicle operation. For example, a rotating shaft within a bearing can result in an electrostatic charge being generated within the vehicle. Objectionable features of the accumulation and discharge of the vehicle's electrostatic charge are well known. Such features include the interference with electronic components grounded to the vehicle (radio static) as well as the annoyance to passengers boarding or standing on the ground and touching the vehicle shortly after it comes to a stop.
The higher the insulating characteristics of the tire the slower the electrostatic voltages are dissipated through the tire. Also, the conductivity of the ground surface is a factor in the rate of electrostatic discharge experienced when the vehicle stops. In general, concrete roadway surfaces dissipate the electrostatic charge much faster than asphalt roadway surfaces. However, driving over a highly conductive metal manhole cover in the roadway will produce a sudden discharge of the electrostatic charge, which is noticable as static in the car radio on the AM frequency. Because rubber is a good insulator, tires are inherently less conductive. Special tire designs are required to continuously discharge the buildup of electrostatic charges during running of the vehicle and, in addition, so that the remaining charge can be quickly dissipated when the vehicle stops.
Tires constructed in the 1940-1970 time period used tread compounds with high volume resistivity values. Electrical charges on these tires as large as 16,000 volts were often recorded. Numerous references show volume resistivity values of the tread rubber as high as 1,000 to 10,000 megohm centimeters. Overall electrical resistivity from the tire rim to the ground surface was disclosed to often exceed 100,000 megohms. Some of the solutions for these resistivity problems of the past are discussed below. Conductivity being the inverse of resistivity, these two terms are both used to characterize a preferred tire having a high conductivity property as well as a low resistivity property.
A simple and effective way to provide a conductive path from the tire rim to the ground through which electrostatic charges can be dissipated is disclosed in U.S. Pat. No. 2,329,332. To decrease the overall electrical resistance from the rim to the ground, a strip or ribbon of a rubber composition compounded to render it electrically conductive is used. This strip is positioned between the carcass and the tread as well as between the carcass and the sidewall. A second strip is also placed circumferentially around the crown portion of the carcass. A chafing strip in the bead area is also disclosed. These strips are not in contact with the ground surface, but are positioned below the tread.
The disclosure of U.S. Pat. No. 2,342,576 shows a number of circumferential layers or strips having high electrical conductivity underlying the tread of the tire. These layers are terminated short of the bead or rim-contacting portions of the tire. The resistivity of the conductive layers is preferably less than 100 ohm centimeter. The tire of this invention is disclosed to eliminate radio static by stopping the flow of electricity by concentrating the charges in these conductive layers.
It has been suggested that electrostatic charges tend to build up on a vehicle's tires as a result of both external and internal friction of the tires. When sufficiently great, the charge results in a spark discharge producing the AM radio interference. In U.S. Pat. Nos. 2,475,199 and 2,930,426 the use of metal wires and cables are incorporated in the construction of the belt package and the carcass of the tire in a manner which prevents unwanted electrostatic charges to accumulate. Electrical current can then be caused to be conducted throughout the tire producing a tire with high conductivity. In this manner the tire uses its reinforcing members to achieve lower resistivity.
To facilitate the dissipation of electrostatic charges, United Kingdom Patent No. 554,216 discloses the treatment of the interior of an inner tube or carcass with a finely-divided non-rubber containing electrically conductive material. The electrically conductive material can be a carbon black or finely divided metallic particles adhering to at least the crown region of the tire. Such a treatment illustrates the use of carbon black as a conductor if sufficient quantities are present in the tire.
As improvements in tire rubber compounding were made the use of tread compounds having highly conductive carbon black components helped solve the initial problems with high resistivity treads of tires. U.S. Pat. No. 2,339,546 discloses that a static electrical charge is often produced on the tire tread because of the friction between the tread and the ground surface. The amount of the charge depends on tire tread design and composition of the tread. Resistivity values for the conventional tread having 30 percent channel black are quoted as being in the order of 1,000 to 10,000 megohm centimeter. However, with an acetylene black to replace the channel black the resistivity values dropped to be only in the order of 10,000 ohm centimeters. This illustrates the historical fact that compounds having highly conductive carbon black components helped solve the high resistivity problems with tires.
The improvements of U.S. Pat. No. 2,339,546 are to overcome the relatively low resistivity of the sidewalls by adding a 1.0 to 2.5 millimeter thick conductive element from a contact point with the rim in the bead portion to extend around or through the tread shoulder and terminate in the face of the tread. The conductive element is made to have a resistivity value of less than 1.0 megohm centimeter. An electrically conductive chafer strip is also used in the bead portion of this tire.
The use of highly conductive sidewall portions in a tire is disclosed in U.S. Pat. No. 5,173,135. The rubber compositions for the purposes of this invention are made to have a volume resistivity not greater than 10,000 ohm centimeter. The purposes for this sidewall rubber is to be resistant to stains and the attraction of mud, dust and dirt pickup during or after running. The carbon black of the sidewall compound includes high abrasion furnace black, fast extrusion furnace black as well as general purpose furnace black.
The problem of high volume resistivity values in tires has again been introduced by the use of high volume resistivity silica-rich compounds in tires. The high silica treads are excellent for low rolling resistance and wear life of the tire. This makes their use most desirable as a tread compound. In U.S. Pat. Nos. 4,519,430; 5,066,721; and 5,227,425 the improvements in tire performance are disclosed with silica-rich treads. U.S. Pat. No. 5,227,425 discloses the compounds and processes for obtaining a tire having a high performance tread for all seasons. This compound is especially desirable in the tread of the tire. However, the volume resistivity of high silica-rich rubber treads can once again result in a weak link in the ability of the tire to control the electrostatic charge on the vehicle. Therefore, the need to once again provide a high conductivity and low resistivity tire exists. Such a need should be consistent with the continued use of low volume resistivity compounds in other portions of the tire.